The present invention relates generally to a super-conductive coil in a synchronous rotating machine. More particularly, the present invention relates to a support structure for super-conducting field windings in the rotor of a synchronous machine.
Synchronous electrical machines having field coil windings include, but are not limited to, rotary generators, rotary motors, and linear motors. These machines generally comprise a stator and rotor that are electromagnetically coupled. The rotor may include a multi-pole rotor core and coil windings mounted on the rotor core. The rotor cores may include a magnetically-permeable solid material, such as an iron-core rotor.
Conventional copper windings are commonly used in the rotors of synchronous electrical machines. However, the electrical resistance of copper windings (although low by conventional measures) is sufficient to contribute to substantial heating of the rotor and to diminish the power efficiency of the machine. Recently, super-conducting (SC) coil windings have been developed for rotors. SC windings have effectively no resistance and are highly advantageous rotor coil windings.
Iron-core rotors saturate at air-gap magnetic field strength of about 2 Tesla. Known super-conductive rotors employ air-core designs, with no iron in the rotor, to achieve air-gap magnetic fields of 3 Tesla or higher, which increase the power density of the electrical machine and result in significant reduction in weight and size. Air-core super-conductive rotors, however require large amounts of super-conducting wire, which adds to the number of coils required, the complexity of the coil supports, and the cost. Such super-conductive rotors have their super-conducting coils cooled by liquid helium, with the used helium cooled being returned as room-temperature gaseous helium. Using liquid helium for cryogenic cooling requires continuous reliquefaction of the returned, room-temperature gaseous helium, and such reliquefaction poses significant reliability problems and requires significant auxiliary power.
High temperature SC coil field windings are formed of super-conducting materials that are brittle, and must be cooled to a temperature at or below a critical temperature, e.g., 27xc2x0 K., to achieve and maintain super-conductivity. The SC windings may be formed of a high temperature super-conducting material, such as a BSCCO (BixSrxCaxCuxOx) based conductor.
SC coil cooling techniques include cooling an epoxy-impregnated SC coil through a solid conduction path from a cryocooler. Alternatively, cooling tubes in the rotor may convey a liquid and/or gaseous cryogen to a porous SC coil winding that is immersed in the flow of the liquid and/or gaseous cryogen. However, immersion cooling requires the entire field winding and rotor structure to be at cryogenic temperature. As a result, no iron can be used in the rotor magnetic circuit because of the brittle nature of iron at cryogenic temperatures.
What is needed is a super-conducting field winding assemblage for an electrical machine that does not have the disadvantages of the air-core and liquid-cooled super-conducting field winding assemblages of, for example, known super-conductive rotors.
In addition, high temperature super-conducting (HTS) coils are sensitive to degradation from high bending and tensile strains. These coils must undergo substantial centrifugal forces that stress and strain the coil windings. Normal operation of electrical machines involves thousands of start-up and shut-down cycles over the course of several years that result in low cycle fatigue loading of the rotor. Furthermore, the HTS rotor winding must be capable of withstanding 25% overspeed operation during rotor balancing at ambient temperature and occasional over-speed at cryogenic temperatures during operation. These overspeed conditions substantially increase the centrifugal force loading on the windings over normal operating conditions.
HTS coils used as the rotor field winding of an electrical machine are subjected to stresses and strains during cool-down and normal operation as they are subjected to centrifugal loading, torque transmission, and transient fault conditions. To withstand the forces, stresses, strains and cyclical loading, the HTS coils must be properly supported in the rotor. These support systems and structures that hold the coils in the rotor should secure the coils against the tremendous centrifugal forces due to the rotation of the rotor. Moreover, these support systems and structures should protect the HTS coils and ensure that the coils do not crack, fatigue or otherwise break.
Developing support systems for HTS coil has been a difficult challenge in adapting SC coils to rotors. Examples of HTS coil support systems for rotors that have previously been proposed are disclosed in U.S. Pat. Nos. 5,548,168; 5,532,663; 5,672,921; 5,777,420; 6,169,353, and 6,066,906. However, these coil support systems suffer various problems, such as being expensive, complex and requiring an excessive number of components. There is a long-felt need for a rotor and coil support system for a HTS coil in a synchronous machine. The need exists for HTS coil support system made with low cost and easy-to-fabricate components.
A rotor having twin HTS coils on a rotor core of a synchronous machine. Similarly, a support structure is disclosed for mounting the pair of HTS coils on the rotor. The rotor may be for a synchronous machine originally designed to include HTS coils. Alternatively, the HTS rotor may replace a copper coil rotor in an existing electrical machine, such as in a conventional generator. The rotor and its HTS coils that are described here in the context of a generator, but the HTS coil rotor is also suitable for use in other synchronous machines.
A dual racetrack HTS coil design for two-pole field winding provides several advantages including simplicity in coil design and in coil support design. In addition, a dual coil design has substantially twice the amount of coil winding of a single-coil rotor. Thus, a dual coil design has a substantially greater capacity for power generation (when the coil is incorporated in a rotor of a generator).
In a first embodiment, the invention is a rotor for a synchronous machine comprising: (i) a rotor core having a rotor axis; and (ii) a pair of super-conducting coil windings mounted on the rotor core, each of said coil windings in a respective plane that is parallel to and offset from the rotor axis.
In another embodiment, the invention is a rotor for a synchronous machine comprising: (i) a rotor core having a rotor axis and recessed surfaces extending longitudinally along the rotor core; (ii) a first and second super-conducting coil windings mounted on the rotor core, each of said coil windings being in a plane that is parallel to and offset from the rotor axis; (iii) a plurality of first tension rods spanning and connecting opposite side sections of each of said coil windings, and (iv) a plurality of second tension rods spanning between and connecting both of the coil windings.